Corrective line quantities for compensating for differences between reading positions in an auxiliary scanning direction of linear image sensors of a 3-line image sensor are determined (in a step S1), and the determined corrective line quantities are divided into integer-part corrective line quantities INT[Cbg], INT[Crg] and decimal-part corrective line quantities DEC[Cbg], DEC[Crg]. The decimal-part corrective line quantities DEC[Cbg], DEC[Crg] are interpolated in real-time by a hardware arrangement from preceding image data and presently read image data, using line buffer memories (6J, 6K) (in a step S21). The integer-part corrective line quantities INT[Cbg], INT[Crg] are changed in addresses when they are stored as a file in a hard disk or the like (in a step S24), thus providing image data which are equalized in reading positions.